1. Field of the Invention
The present invention relates to fabricating light emitter packages and optical elements and, more specifically, to primary optics and packages for LED devices and methods of manufacturing such optics and packages.
2. Description of the Related Art
Light emitting diodes (LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED. The useful light is generally emitted in a hemispherical emission pattern in the direction of the LED's top surface, typically resulting in a predominantly Lambertian light emission profile. This confined emission is not generally suitable for many applications such as those requiring specific beam shaping; for example, collimated beam profiles, dispersed beam profiles, or specialized beam profiles.
In order to facilitate the use of LEDs in various applications, LEDs are typically arranged in a package which encases and protects the LED and provides electrical and thermal contact as well as enhances the emission of light from the LED.
LED packages typically incorporate some type of encapsulant such as a transparent epoxy or silicone material surrounding the LED chip to enhance light extraction from the chip and protect the chip and related contacts structure (e.g. wire bonds) from exposure to physical damage or environmental conditions which could lead to corrosion or degradation.
LED packages may also incorporate a ‘primary’ optical element to enhance light extraction from the package and in some instances to provide output light beam shaping by controlling the angle-dependent emission properties of the lamp. The primary optical element typically has a predominantly hemispherical profile and may be formed by shaping the encapsulant (e.g., by a molding process), or may comprise a separately fabricated optical element (e.g., a molded glass or silicone lens) that may be mounted to the package—typically in contact with the encapsulant material.
LEDs intended for lower-power applications such as signal or indicator lights are often arranged in packages that can include a molded or cast plastic body that encapsulates an LED chip, a lens, and conductive traces or leads. Heat is typically generated by LEDs when power is applied and the LEDs are emitting light. The traces or leads serve as a conduit to supply the LED chip with electrical power and can also serve to draw heat away from the LED chip. In some packages, a portion of the lead frame extends out of the package for connection to circuits external to the lead frame package.
For current state-of-the-art LED packages intended for lighting applications, a common configuration is the “surface mount” package which incorporates one or more LED chips mounted onto a planar substrate. Such packages typically include an appropriate encapsulant material (e.g., silicone) as well as a primary optic element. In some cases, the primary optic element may be produced by forming the encapsulant material into a specified shape as it is added to the package—for example by molding the silicone encapsulant into a hemispherical shape using a process such as injection molding. Alternately, a separate glass or molded silicone lens may be applied to the package. For surface mount packages, which typically require high temperature (200-300° C.) solder reflow processing to attach the LED package to its final fixture, the possible lens and encapsulant materials typically include silicones and glasses.
In surface mount packages where a separate lens is applied, the lens typically comprises a solid hemisphere with a substantially flat or planar surface which is mounted above the LED chip to allow clearance for the LED chip wire bonds. This requires that the LED chip be placed below the origin of the hemisphere and may necessitate a supportive element which holds the lens at a specified height above the chips and “retains” the lens by preventing lateral deflection relative to the LEDs. For a supportive element's retention features to retain the lens against lateral forces, it may be necessary for a retaining feature to rise above the bottom surface and surround the lower portion of the lens. In the package described in U.S. Patent Application Publication No. 2004/0079957 to Loh, the hemispherical lens sits within a recessed lip of the reflector plate. For various cost and fabrication reasons, this retaining feature is typically not transparent to light but rather is reflective. This arrangement can result in some of the light emitted by the LED chip being lost due to loss mechanisms such as total internal reflection. Further, because the LED chip sits below the bottom surface of the hemispheric lens, additional reflective surfaces are required to direct sideways emitted LED light to the lens and out the package. This reflection process is not 100% efficient, resulting in additional loss of light. Also, reflections from these surfaces effectively create a larger, more complex light source (compared, for example, to the chip alone) which can require more complex secondary optics that can result in additional light loss.
By nature, the primary optical elements and associated encapsulant materials of surface mount LED packages typically surround or encapsulate one or more LED chips and any associated electrical contacts. The preferred geometry for the primary optical element 10 has been a predominantly hemispherical shape, as shown in FIGS. 1a-1e. This shape has two primary benefits: (1) if large enough relative to the LED source, most of the light emitted by the LED is incident on the optic surface with a path that is nearly parallel to the surface normal (since the optic is typically surrounded by air and has an index of refraction higher than air, this minimizes the possibility of total internal reflection and hence efficiency loss), and (2) hemispherical shapes are readily fabricated onto planar surfaces by conventional molding processes. If the diameter of the hemispherical optic is not large compared to the largest dimension of the associated LED source, the package may suffer from output losses caused by total internal reflection (“TIR”) within the optic, resulting in a low package extraction efficiency.
While the hemispherical optic 10 geometry is desirable with respect to efficiency and ease of fabrication, this geometry typically does little to modify the initial optical output beam profile from the LED source. As a result, the light output from the package may be predominantly Lambertian in nature, as shown in FIGS. 2A and 2B—similar to that emitted by a typical LED. In order to achieve a substantially non-Lambertian light beam profile (e.g., collimated, dispersed, or otherwise shaped) from a primary optic centered over a predominantly Lambertian emitting source such as a typical LED or array of LEDs, it is generally necessary to utilize a more complex optical geometry. However, many such geometries are not readily fabricated by molding processes. Specifically, since the mold cavity must be removed from the substrate surface following curing of the molded optic, it is not generally possible to mold parts which have ‘overhangs’, are tapered, or are narrower at the base (near the substrate) than at the top.
As illustrated in FIGS. 1c-e, in traditional molding, a mold 108 is applied to a planar substrate 100 with associated LED chips 102. The cavities in the mold are filled with a suitable encapsulating/optical material such as silicone or epoxy. The encapsulant is then at least partially cured, and the mold removed, leaving behind encapsulant on the surface in the form of a primary optical element. In order to remove the mold from the primary optic after at least partial curing, it is necessary that there are no regions of ‘overhang’ which would prohibit mold removal. This limitation in particular can inhibit the molding of many collimating-type optics. While there are molding techniques which can allow such geometries, they typically involve complex molds with moving parts which are not suitable for batch fabrication of many molded elements in an array on one surface. Undercuts on optics require mold pieces to pull out or separate in a lateral direction (e.g., parallel to the substrate surface). These mold types, called side-action molds, are thus not well suited to the fabrication of dense arrays of optics on a substrate as the lateral motions of adjacent lenses would interfere. While individual (rather than batch) molding of complex optics using side-action molds may be possible, molding LED package optics one-at-a-time is generally not feasible due to the associated high manufacturing cost and low throughput.
Alternative approaches to the fabrication of primary optics on LED packages are also generally not suitable for the batch formation of complex optic shapes. For example, predominantly hemispherical shapes may be achieved by a dispensing process. However, the shape of such optics is determined primarily by dispensed mass, surface tension, and gravity, with little flexibility for forming specific non-hemispherical shapes such as those with an “undercut” feature. Similarly, approaches utilizing a primary optic element which is molded separately and then attached to the LED package can be limited by the need to place the optic above the surface of the LEDs and associated electrical contacts and also provide stable attachment to the package. Such an approach would suffer from cost, efficiency, and manufacturability issues compared to the more simple molding process.
As a result of these limitations, beam shaping for LED packages is typically achieved through the use of “secondary” optics. Such secondary optics generally increase overall cost and reduce efficiency. Further, the shape of the secondary optic can be limited by the size and geometry of the primary LED optic or lens—this can further reduce efficiency and limit the potential for beam shaping in some applications, particularly those involving collimation of the LED light, where it is helpful to bring the optical element as close to the light source (LED chip or chips) as possible. The use of secondary optics can result in lighting solutions which are bulky, require additional design work and alignment, optical loss, and additional costs. In addition, the materials commonly used in the fabrication of secondary optics (e.g., plastics, polycarbonate, PMMA or glass) can result in elements which are costly, heavy, and less stable with respect to degradation when subjected to the heat and high intensity light associated with lighting-class LED packages.
Additionally, depending on the application, there may be cases where a secondary optical element is required (e.g., if the required light beam profile necessitates an optical element which is too large to conveniently or economically fabricate as a primary optical element on a LED package). In such cases, the ability to fabricate a more complex primary optic may still find application in that the geometry of the primary optic may be tailored to produce an optical beam profile which reduces the constraints on the design of the secondary optical element, thereby enabling, for example, a lower cost or less bulky, more simplified secondary optic and reducing overall system costs.